Nucleic acid analysis chip integrating a waveguide and optical apparatus for the inspection of nucleic acid probes

ABSTRACT

A chip for nucleic acid analysis includes a body (2, 9), in which a detection chamber (7) is formed for accommodating nucleic acid probes (12, 12′). A waveguide (8) is integrated in the body (2, 9) is and is arranged at the bottom of the detection chamber (7) so that an evanescent wave (EW), produced at an interface (8a) of the waveguide (8) when a light radiation is conveyed within the waveguide (8), is irradiated towards the inside of the detection chamber (7). An apparatus for inspection of nucleic acid probes includes: a holder (22), on which a chip (1) for nucleic acid analysis is loaded, the chip containing nucleic acid probes (12, 12′); a light source (24) for supplying an excitation radiation to the nucleic acid probes (12, 12′); and an optical sensor (25) arranged so as to receive radiation coming from the nucleic acid probes (12, 12′).

TECHNICAL FIELD

The present invention relates to a chip for nucleic acid analysis, which integrates a waveguide, and to an optical apparatus for inspection of nucleic acid probes.

BACKGROUND ART

As is known, the analysis of nucleic acids requires, according to different modalities, preliminary steps of preparation of a sample of biological material, of amplification of the nucleic material contained therein, and of hybridization of individual target or reference strands, corresponding to the sequences sought. Hybridization occurs (and the test yields a positive outcome) if the sample contains strands complementary to the target strands.

At the end of the preparatory steps, the sample must be examined to control whether hybridization has occurred (the so called detection step). For this purpose, various inspection methods and apparatuses are known, for example of an optical or electrical type. In particular, the methods and apparatuses of an optical type are frequently based upon the phenomenon of fluorescence. The reactions of amplification and hybridization are conducted so that the hybridized strands, contained in a detection chamber made in a support, include fluorescent molecules or fluorofors (the hybridized strands may be either grafted to the bottom of the detection chamber or remain in liquid suspension). The support is exposed to a light source having an appropriate spectrum of emission, such as to excite the fluorofors. In turn, the excited fluorofors emit a secondary radiation at an emission wavelength higher than the peak of the excitation spectrum. The light emitted by the fluorofors is collected and captured by an optical sensor. In order to eliminate the background light radiation, which represents a source of disturbance, the optical sensor is provided with band-pass or interferential filters centred at the wavelength of emission of the fluorofors.

However, the difference between the maximum peak of the emission spectrum of the fluorofors and the peak of the excitation spectrum (also referred to as “Stokes shift”) is not very high, and the filters, however selective they may be, can only attenuate the light emitted by the source and subsequently diffused, without, however, eliminating it altogether. It should also be taken into account that the materials used for providing the supports often have high reflecting power. For example, microfluidic devices for the analysis of nucleic acids integrated in semiconductor chips are increasingly widespread. In integrated microfluidic devices, the detection chamber often has the bottom coated with a layer of silicon dioxide and, sometimes, also metal electrodes are present, for example of gold or aluminium. In effect, hence, only a relatively small part of the light emitted by the source is absorbed, whereas a conspicuous fraction is reflected and is potentially capable of disturbing the detection of the light emitted by the fluorofors.

DISCLOSURE OF INVENTION

The aim of the present invention is to provide a chip for analysis of nucleic acids and an optical apparatus for the inspection of nucleic acid probes that will enable the limitations described to be overcome.

According to the present invention, a chip for analysis of nucleic acids and an optical apparatus for the inspection of nucleic acid probes are provided, as defined in claims 1 and 13, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, an embodiment thereof is now described, purely by way of non-limiting example and with reference to the attached plate of drawings, wherein:

FIG. 1 is a top plan view of a chip for analysis of nucleic acids in accordance with a first embodiment of the present invention;

FIG. 2 is a cross-sectional view through the chip of FIG. 1, taken according to the line II-II of FIG. 1;

FIG. 3 shows an enlarged detail of the view of FIG. 2;

FIGS. 4 and 5 show a first variant and a second variant of the chip of FIG. 1;

FIG. 6 is a simplified block diagram of an optical inspection apparatus for the detection of nucleic acids, which uses the chip of FIG. 1;

FIG. 7 is a schematic illustration of a detail of the chip of FIG. 1 loaded into the apparatus of FIG. 6;

FIG. 8 shows a distribution of intensity associated to a light radiation propagating in the chip of FIG. 1;

FIG. 9 is a cross-sectional view through a chip for nucleic acid analysis according to a second embodiment of the present invention;

FIG. 10 is a simplified block diagram of an optical inspection apparatus for detection of nucleic acids which uses the chip of FIG. 9; and

FIG. 11 is a schematic illustration of a detail of the chip of FIG. 9 loaded into the apparatus of FIG. 10.

BEST MODE FOR CARRYING OUT THE INVENTION

FIGS. 1 and 2 show a chip 1 in which a chemical microreactor for the analysis of nucleic acids (here DNA) is provided. The chip 1 comprises: a substrate 2 made of semiconductor material; inlet reservoirs 4; a plurality of microfluidic channels 5; heaters 6 associated to the microfluidic channels 5; a detection chamber 7; and a waveguide 8.

More precisely, the inlet reservoirs 4 and the detection chamber 7 are defined in a structural layer 9 arranged on the surface of the substrate 2 (for example, the structural layer 9 may either comprise a resist layer deposited on the substrate 2 or a glass chip glued thereto).

The microfluidic channels 5 are buried within the substrate 2, for example as described in EP-A-1 043 770, in EP-A-1 130 631 or in the published patent application US 2005/282221, and extend between the inlet reservoirs 4 and the detection chamber 7. Furthermore, the microfluidic channels 5 are fluidly coupled both to the inlet reservoirs 4, through inlet openings 10, so as to be accessible from outside, and to the detection chamber 7, through outlet openings 11.

The heaters 6, here including resistive elements made of polysilicon, are formed on the surface of the substrate 2 and extend in a direction transverse to the microfluidic channels 5. Furthermore, the heaters 6 are electrically connectable in a known way to external electric power sources (not shown) and can be driven to release thermal power to the microfluidic channels 5 so as to cyclically control the temperature within them according to predetermined thermal profiles.

The detection chamber 7 is designed to receive a fluid containing previously processed nucleic material in suspension, to perform a step of optical detection of nucleic acid sequences. As illustrated in greater detail in FIG. 3, the detection chamber 7 is formed above the waveguide 8 and accommodates a plurality of so-called “DNA probes” 12, comprising single-stranded reference DNA containing predetermined sequences of nucleotides. More precisely, the DNA probes 12 are arranged in predetermined positions so as to form an array and are grafted directly to the waveguide 8, which forms the bottom of the detection chamber 7. After a step of hybridization, some of the DNA probes, designated by 12′, are hybridized, i.e., they are bound to individual complementary DNA sequences, and contain fluorofors 15.

The waveguide 8 is formed on the substrate 2 and extends at least under the array of DNA probes 12, 12′ and, preferably, at the bottom of the entire detection chamber 7. Furthermore, the waveguide 8 comprises a plane layer of optically conductive material having a predetermined primary refraction index N₁. For example, silicon oxynitride or silicon dioxide may be used, appropriately doped with germanium, phosphorus or boron (the value of the primary refraction index N₁ depends in fact upon the concentration of the dopant species). Alternatively, it is possible to diffuse titanium, obtaining a guiding effect by ion exchange.

The waveguide 8 is defined on one side by a first interface 8 a, with the inside of the detection chamber 7, and on an opposite side by a second interface 8 b, with the substrate 2. The primary refraction index N₁ is chosen so that the energy associated to electromagnetic radiation conveyed in the waveguide 8 will in part remain confined inside it, with the exception of a predetermined fraction or tail, which leaves the waveguide (evanescent wave). More precisely (see also FIG. 8), the primary refraction index N₁ is selected in relation to a secondary refraction index N₂, which is characteristic of the fluid introduced into the detection chamber 7 for the detection step, so that the electromagnetic energy exiting through the first interface 8 a (energy of the evanescent wave EW) is a predetermined fraction of the total. Furthermore, the relation between the primary refraction index N₁ and the secondary refraction index N₂ is such that the intensity of the evanescent wave EW becomes substantially negligible at an attenuation distance D₀ of approximately 1-2 μm from the first interface 8 a.

Preferably, the ratio between the primary refraction index N₁ of the waveguide 8 and a refraction index of the substrate 2 is such as to prevent substantially the dispersion of energy through the second interface 8 b.

As illustrated in the detail of FIG. 3, a face 1 a of the chip 1, designed to be exposed to a light source of external excitation, is cut so as to form a predetermined angle with respect to the first interface 8 a and the second interface 8 b. An outer edge of the waveguide 8 is exposed on the face 1 a and defines an optical coupling surface 8 c for conveying visible electromagnetic radiation from outside in the direction of the waveguide 8.

In the variant illustrated in FIG. 4, the DNA probes 12 are grafted to an anchorage layer 14 deposited on the waveguide 8. In this case, the first interface 8 a is defined between the waveguide 8 and the anchorage layer 14. The primary refraction index N₁ is determined taking into account also the refraction index and the thickness of the anchorage layer 14. In practice, the attenuation distance of the evanescent wave (from the first interface 8 a) is approximately 1-2 μm greater than the thickness of the anchorage layer 14.

In the further variant illustrated in FIG. 5, the light guide 8 comprises a core 8 d, having the primary refraction index N₁, and a cladding 8 e, which coats both of the faces of the core 8 d and has a secondary refraction index N₂. In this case, the first interface 8 a is defined between the core 8 d and the cladding 8 e, on the side to which the DNA probes 12 are anchored. Preferably, the cladding 8 e is thinned out where the DNA probes 12 are anchored, to favour excitation of the fluorofors 15 that may possibly present. If need be, an anchorage layer for the DNA probes 12 (here not shown) can be envisaged.

The microreactor integrated in the chip 1 is prearranged for performing reactions of amplification of nucleic material, for example by PCR (Polymerase Chain Reaction) and hybridization of the DNA probes 12. For this purpose, a biological sample in liquid suspension, containing nucleic material previously treated, is supplied to the inlet reservoirs 4 and fed into the microfluidic channels 5. Here, the sample is subjected to thermal cycling in order to amplify the DNA present in a known way. At the end of the amplification step, the biological sample is further made to advance as far as the detection chamber 7, where the DNA probes 12 are located. If the biological sample contains sequences of nucleotides complementary to the DNA probes 12, the latter are hybridized. Furthermore, the amplification reactions are conducted so that the hybridized DNA probes 12′ will contain fluorofors 15 (shown only schematically) having a characteristic emission wavelength.

With reference to FIGS. 6 and 7, the number 20 designates an optical inspection apparatus for the detection of hybridized DNA strands, based upon fluorescence. The inspection apparatus 20 comprises a control unit 21, a holder 22 for housing an item of the chip 1, a light source 24 and an optical sensor 25, provided with a collimation and focusing device 26, and a filter 27, having passband centred around the wavelength of emission characteristic of the fluorofors 15.

Furthermore, the control unit 21 comprises a compensator module 28, which receives digital images IMG from the optical sensor 25, and an image processing module 30.

FIG. 7 shows a detail of the chip 1 loaded into the holder 22 so as to be examined. The light source 24 emits electromagnetic radiation with a spectrum such as to excite the fluorofors 15 and may comprise a laser (coherent monochromatic source) or else a conventional incandescent lamp or a LED, with appropriate optical filters (incoherent source). When the chip 1 with the integrated microreactor is located in the holder 22 in a reading position, the light source 24 is optically coupled to the waveguide 8. More precisely, the light source 24 is arranged so that the light emitted is directed towards the face 1 a of the chip 1. Furthermore, the orientation of the light source 24 is such that the radiation incident on the optical coupling surface 8 c of the waveguide 8 is conveyed along the waveguide 8 itself.

The optical sensor 25, for example of a CMOS or CCD type, is arranged so as to collect the light emitted by the fluorofors 15 present in the detection chamber 7 of the microreactor integrated in the chip 1. In practice, when the chip 1 is loaded into the holder 22, the optical sensor 25 is substantially parallel to the first interface 8 a, at a detection distance D_(S) much greater than the attenuation distance D₀, for example approximately 3-7 cm.

The inspection apparatus 20 operates in the way described hereinafter. Initially, an item of the chip 1, integrating a microreactor in which a step of hybridization of the DNA probes 12 has been carried out, is loaded into the holder 22. The control unit 21 activates the light source 24, and part of the excitation radiation emitted is conveyed along the waveguide 8 through the optical coupling surface 8 c.

As shown schematically in FIG. 8, a part of the energy associated to the light radiation remains confined within the waveguide 8. However, a fraction of energy depending upon the ratio between the primary refraction index N₁ and the secondary refraction index N₂ exits through the interface 8 a and gives rise to an evanescent wave EW outside the waveguide 8. The intensity of the evanescent wave EW decays exponentially as the distance D from the first interface 8 a increases and becomes substantially negligible within the attenuation distance D₀. The fluorofors 15 of the hybridized DNA probes 12′ are in any case located at a distance from the first interface 8 a that is much shorter than the attenuation distance D₀ (normally, less than 100 nm). The energy of the evanescent wave EW is hence sufficient to excite the fluorofors 15, which emit light that can be detected by the optical sensor 25, at the characteristic wavelength. The optical sensor 25 is not, however, able to detect the radiation due to the evanescent wave EW, because the detection distance D_(S) (5-7 cm) is much greater than the attenuation distance (1-2 μm). In other words, the intensity of the evanescent wave EW is substantially zero at the detection distance D_(S) from the first interface 8 a, where the optical sensor 25 is located.

The digital images IMG detected by the optical sensor 25 are sent to the compensator module 28 of the control unit 21, which balances the brightness level to compensate for the attenuation of the light radiation (and of the evanescent wave EW) along the waveguide 8 as the distance from the optical coupling surface 8 c increases (the most distant fluorofors 15 are excited to a lesser extent on account of attenuation and might not be recognized correctly). The compensator module 28 supplies compensated images IMG_(C) to the image processing module 30, which is designed for detection of the hybridized DNA probes 12′, containing fluorofors 15, and of their position in the array.

The chip and the inspection apparatus described substantially enable elimination of the disturbance due to the excitation radiation of the fluorofors during the optical detection of hybridized DNA probes. The excitation radiation is in fact almost entirely confined within the waveguide, and the evanescent wave that exits in the direction of the detection chamber to excite the fluorofors is attenuated at a very short distance from the waveguide itself and does not reach the optical sensor. Advantageously, the inspection apparatus requires just one waveguide, since the radiation emitted by the fluorofors is collected directly by the optical sensor. The integration of the waveguide within the chip represents a further advantage of the invention. On the one hand, in fact, the geometry of the waveguide can be optimized so that the evanescent wave will have exactly the desired intensity. On the other hand, the integrated waveguide can be optically coupled to the excitation light source in a simple and precise way, and the microfluidic device is suited to carrying out the step of detection in a completely automatic way.

FIGS. 9 and 10 show a different embodiment of the invention. In this case (FIG. 9), a chip 100 integrates a chemical microreactor that comprises a substrate 102 made of semiconductor material, inlet reservoirs 104, microfluidic channels 105 buried in the substrate 102, heaters 106, a detection chamber 107, and a waveguide 108.

The inlet reservoirs 104 and the detection chamber 107 are defined in a structural layer 109 arranged on the surface of the substrate 102 and are fluidly coupled to the microfluidic channels 105, as already explained with reference to FIGS. 1 and 2. As may be seen in FIG. 9, the substrate 102 projects beyond the structural layer 109 on the side of the detection chamber 107.

The detection chamber 107 is formed above the waveguide 108 and accommodates a plurality of DNA probes 112, arranged in predetermined positions so as to form an array. The DNA probes 112 are moreover grafted to the waveguide 108, which forms the bottom of the detection chamber 107. Hybridized DNA probes 112′ contain fluorofors 115.

The waveguide 108 comprises a plane layer of optically conductive material having a predetermined primary refraction index N₁, for example appropriately doped silicon oxynitride or silicon oxide. Furthermore, the waveguide 108 is formed on the substrate 102 and extends at the bottom of the array of DNA probes 12 and also outside the detection chamber 7, for example in a direction opposite to the microfluidic channels 105. A portion of the waveguide 108 laterally external to the structural layer 109 defines an optical coupling surface 108 c. Preferably, the optical coupling surface 108 c is free and arranged directly facing outwards.

Illustrated in FIG. 10 is an inspection apparatus 120 of an optical type, which comprises a control unit 121, a holder 122 for housing an example of the chip 100, a light source 124, an optical coupling element 123, and an optical sensor 125, which is provided with a collimation and focusing device 126 and an interferential filter 127, having a passband centred around the wavelength of emission characteristic of the fluorofors 115.

Furthermore, the control unit 121 comprises a compensator module 128, which receives digital images IMG from the optical sensor 125, and an image processing module 130.

As shown in FIG. 11, the optical coupling element 123 comprises an optical prism 131 having an input surface 131 a, which receives the radiation emitted by the light source 124, and an output surface, which faces the optical coupling surface 108 a of the chip 100 when it is loaded into the holder 122. The optical prism 131 is configured so as to convey within the waveguide 108 the radiation coming from the light source 124.

Finally, it is evident that modifications and variations may be made to the chip and to the apparatus described herein, without departing from the scope of the present invention, as defined in the annexed claims. For example, the waveguide can be made of any material and using any technique, in particular those suited to being integrated in the processes of fabrication of semiconductor devices. The hybridized DNA probes may be in liquid suspension, instead of immobilized at the bottom of the detection chamber. In this case, only the fluorofors included in DNA probes that are located in the immediate neighbourhood of the waveguide, within the attenuation distance, are excited. The optical prism for coupling to the light source, when present, can be integrated in the chip (for example bonded), instead of forming part of the detection apparatus. Finally, the chip may also comprise just the detection chamber, in addition to the waveguide. 

1. A chip for nucleic acid analysis, comprising: a substrate made of semiconductor material; a detection chamber for accommodating nucleic acid probes; a plurality of microfluidic channels buried in said substrate and fluidly coupled to said detection chamber; a plurality of heaters connectable to external electric power sources for controllably releasing thermal power to said microfluidic channels; and a waveguide formed on said substrate and set adjacent to said detection chamber, wherein said detection chamber is defined in a structural layer above said waveguide, so that an evanescent wave will extend from an interface of said waveguide into said detection chamber when an excitation radiation is conveyed within said waveguide.
 2. The chip according to claim 1, wherein said waveguide is a plane waveguide.
 3. The chip according to claim 2, wherein said waveguide comprises a plane layer made of a material selected from the group consisting of: silicon oxide and silicon oxynitride.
 4. The chip according to claim 3, wherein said waveguide contains a dopant species with a predetermined concentration, said dopant species being selected from the group consisting of: phosphorus, germanium, boron, and titanium.
 5. The chip according to claim 1, wherein said waveguide comprises an optical coupling surface optically accessible from outside.
 6. The chip according to claim 5, wherein said optical coupling surface forms a predetermined angle with respect to said interface so that radiation incident from outside on said optical coupling surface is conveyed within said waveguide.
 7. The chip according to claim 5, wherein said waveguide and said waveguide extends laterally outside said structural layer so that a portion of said waveguide laterally external to the structural layer defines said optical coupling surface.
 8. The chip according to claim 1, wherein said nucleic acid probes are grafted to said waveguide.
 9. The chip according to claim 8, wherein said waveguide comprises a core and a cladding coating opposite faces of said core, said cladding being thinned out where said nucleic acid probes are grafted.
 10. An optical apparatus for the inspection of nucleic acid probes, comprising: a holder for housing a chip for nucleic acid analysis, containing nucleic acid probes; a light source for supplying an excitation radiation to said nucleic acid probes; an optical sensor arranged so as to receive radiation coming from said nucleic acid probes; and a chip for nucleic acid analysis according to claim 5 loaded into said holder.
 11. The apparatus according to claim 10, wherein said waveguide is coupled to said light source through said optical coupling surface.
 12. The apparatus according to claim 11, wherein said light source is arranged so as to convey the excitation radiation directly towards said optical coupling surface.
 13. The apparatus according to claim 11, comprising an optical coupling element arranged between said light source and said optical coupling surface.
 14. The apparatus according to claim 13, wherein said optical coupling element comprises an optical prism having an input surface for receiving the excitation radiation from said light source and an output surface facing said optical coupling surface.
 15. The apparatus according to claim 10, comprising a control unit for receiving digital images from said optical sensor.
 16. The apparatus according to claim 15, wherein the control unit comprises a compensator module for balancing a level of brightness in said digital images to compensate for the attenuation of the excitation radiation and of said evanescent wave along said waveguide as a distance from said optical coupling surface increases. 